For years, packaging of food products for ambient temperature storage and distribution has been primarily conducted in glass or metal containers. The costs associated with these containers, their closures, and their labels have increased and are projected to continue to rise. Utilization of thermoplastic materials would conserve more energy on a packaging and distribution systems basis. Additionally, the costs would not be as great. Therefore, it would be extremely desirable to provide more food products packaged in thermoplastic material.
Additionally, in the processing of foods there have generally been two approaches to the packaging of food products in a container. The first method utilizes retorting, whereby a food product is placed in a container, the container is sealed and then the product and container are subjected to heat, such that the product is sterilized. In the second method, a container is subjected to a sterilizing process prior to its receiving sterilized food product. Conventional processes for sterilizing containers in which food stuff are subsequently packaged include UV irradiation, treatment with a mixture of steam and air, and an aseptic technique in which the interior wall of the container is sprayed with hydrogen peroxide and subsequently dried.
It can be appreciated that because of the heat sensitivity of most thermoplastics, conventional retort sterilization techniques can damage or destroy most thermoplastic packages of the type capable of containing food product under ambient conditions for prolonged periods. On the other hand, the application of aseptic packaging would permit increased usage of thermoplastic packaging materials, since application of aseptic techniques would leave the package unaltered and undamaged. Additionally, a food product subjected to aseptic packaging incurs minimal alteration due to processing, thereby theoretically yielding a higher quality-end product.
Experience has shown hydrogen peroxide to be a particularly reliable sterilizing agent for killing micro-organisms in food product containers. The germicidal action of peroxide depends upon the formation of the hydrogen peroxide free radical in the formation of free oxygen. The free oxygen being formed during thermal decomposition of H.sub.2 O.sub.2 exhibits a particularly strong sterilizing affect at the moment of formation. The efficiency of the wet aseptic process is attributable to the hydrogen peroxide being able to penetrate the cell walls of microorganisms.
Hydrogen peroxide is a chemical irritant. It is therefore necessary to obtain residual levels of less than 0.5 ppm (parts per million) before contact with the food product. The removal of the peroxide introduced into the food product container is typically achieved by evaporation, and thus an aseptic process requires the heating of the food product container after the introduction therein of the peroxide.
U.S. Pat. No. 4,742,667 discloses a method and an apparatus for aseptic packaging. The general components of the system taught in that particular patent are typical of the components in many aseptic packaging systems. In operation, a food product container arrives at a sterilization station. Hydrogen peroxide or another suitable disinfectant is applied to the inner surface of the container. Oftentimes, the application involves atomization of a liquid so that a mist is applied. After application of the disinfectant, the container is transported along a conveyor, during which transport the food product container is subjected to additional sprayings of hot air.
The amount of disinfectant, the temperature of the initial disinfectant spray, the temperature of the air and the volume of the air all affect the killing of any micro-organisms which may be present inside of the container. Since the residual peroxide concentration must be greatly reduced prior to introduction into the food product container of actual food product, the temperature and the air flow rate are critical. Typically a plurality of air heaters are utilized in drying the food product containers.
An example of an aseptic packaging machine or filler is the type manufactured by FMC Corporation, and known as Metal Box, a former division of CMB Engineering Group, PLC. These aseptic packaging systems utilize an air heater fabricated with bare heater element wire wound around ceramic insulator blocks. The heater element wire comprises nickel and chrome. The heater element is contained within an air supply tube.
This type of air heater has several drawbacks. First, over time, the nickel chromium wire tends to oxidize. The oxide, due to the passage thereover of air, tends to flake and create the possibility of contamination of the ultimately packaged food product. The problem is exacerbated by the fact that the oxide has been linked to being a carcinogen. Another disadvantage is the fact that the passage of air over the ceramic insulator blocks in the presence of the intense heat generated by the bare heater element wire causes the degradation of the ceramic material. Actual usage of the prior art air heater has resulted in the actual loss of ceramic material, which most likely ended up being introduced into the area in the aseptic packaging system where the hot air is introduced into the food product containers. This presents the possibility of particulate contamination of the packaging prior to filling.
One other drawback associated with the prior art air heater is the fact that while the temperature of the air upon impact with the food product containers may need to be in the range of 71.degree.-82.degree. C. (160-180.degree. F.), this requires the heater element wire to have a surface temperature of well in excess of 538.degree. C. (1000.degree. F.). In some sterilization operations, it became necessary to increase the temperature of the heater element wire beyond the recommended temperature.
Finally, the prior art air heater elements also do not provide an easy and effective means for adjusting the air flow through the air heater. While some air heaters would permit adjustment of air flow, it typically became necessary to remove the heater from the system in order to accomplish the adjustment. This creates problems with the maintenance of a sterile environment within the aseptic packaging system.
It is thus apparent that the need exists for an improved air heater which permits the adjustment of air flow and eliminates the possibility of both particulate and chromium oxide contamination. It is also apparent that the need exists for an air heater which can be used in association with an aseptic packaging system to permit adjustments to the rate of air flow to maintain the sterile environment of the sterilization system.